The increased demands put on aircraft flight crew as a result of more complex technology, ever increasing aircraft traffic, and increased demands for safety has brought about a requirement for monitoring aircraft traffic in the vicinity of a surveillance aircraft. Such monitoring includes the automatic identification of potential threats to a surveillance aircraft monitoring target aircraft in such vicinity. As a result, target aircraft have transponders which in response to appropriate interrogation signals return reply signals which may provide information with respect to the range, altitude and bearing of the target aircraft. Certain traffic control system transponders, such as the Mode S systems include unique aircraft identifiers so that each target aircraft is interrogated separately and each reply is stamped with the identity of the target aircraft. This significantly simplifies surveillance processing by the surveillance aircraft.
In systems such as an Air Traffic Control Radar Beacon System (ATCRBS), which do not include unique aircraft identification information in replies to interrogation signals, the determination of aircraft tracks representative of target aircraft from replies is more difficult. The information obtained by periodic interrogation of target aircraft during surveillance periods from the replies provided by the target aircraft are subjected to algorithms to provide a target aircraft track. Once the track is identified and initialized, then the track can be updated and monitored to determine if the target aircraft is a threat to the surveillance aircraft.
Track determination is complicated for several reasons, generally involving spurious target replies. For example, with reference to FIG. 1, a surveillance aircraft 10 can transmit an interrogation signal 16 to a target aircraft 12, whereupon a transponder in the target aircraft 12 provides a first reply signal 18. The delay between the transmission of the interrogation signal and the reception of the reply signal provides range information concerning the distance of the surveillance aircraft from the target aircraft. However, the interrogation signal 16 can result in a second reply signal 19 that is reflected from the ground 14. The second reply signal 19, reflected once from the ground 14, is generally referred to as a single reflection multipath reply. Because the length of time for the travel of the second reply signal is longer than the first reply signal 18, the second reply signal 19 can be interpreted as being from a separate target aircraft at a greater range from the surveillance aircraft. A single reflection multipath reply also can be generated from the interrogation signal 17 being reflected off the ground 14 combined with the direct reply 18 to surveillance aircraft 10 from target aircraft 12. Since the path length is the same as the previous case, where the reply is reflected and the interrogation is direct, the range is the same. Similarly, the interrogation signal 17 can reflect off the ground 14, activate the transponder of the target aircraft 12 which provides a reply signal 20 that also reflects off the ground 14. In this instance, since both the interrogation signal 17 and the reply signal 20 are each reflected off the ground 14, this reply is referred to as a double reflection multipath reply. These double reflection multipath replies will be interpreted by the surveillance aircraft 10 as a target aircraft 12 at an even greater range than indicated by the direct or single reflection multipath replies.
In these situations, a single target aircraft is providing the surveillance aircraft 10 with a plurality of target responses during each interrogation. Thus, from a single surveillance period, consisting of multiple interrogations, multiple replies can be received from a single target aircraft. Such multiple replies may result in multiple tracks as shown in FIG. 2, where a direct reply track 101-103, a single reflection multipath track 104-106, and double reflection multipath 107-109 are indicated. Tracks can also be formed on mixtures of reply types, such as for a combination of single and double reflection replies. In addition, tracks can result from false replies due to electromagnetic interference or other effects such as ATCRBS transponder replies from insufficiently suppressing Mode S transponders.
As a result of the number of tracks which can be interpreted from the various replies from the target aircraft 12, a single target aircraft may have multiple tracks associated therewith. Such multiple tracks may be associated with a single ATCRBS transponder equipped target aircraft, however, will not occur with regard to a Mode S transponder equipped target aircraft as the Mode S equipped target aircraft has unique identification which prevent such multiple tracks from occurring. However, Mode S equipped target aircraft sometimes answer interrogations intended for ATCRBS equipped target aircraft and this may lead to ATCRBS replies in addition to the Mode S reply; thus forming duplicate tracks for a single Mode S equipped transponder target aircraft.
The multiple tracks for a single target aircraft may be referred to as split tracks. Such split tracks for a single target aircraft may be the result of multipath false replies. Split tracks may be displayed for an aircraft flight crew by a Traffic Alert and Collision Avoidance System (TCAS). Display symbols representative of target aircraft displayed as a result of one or more tracks of a split track results in excess clutter on the display and may also lead to excess aural messages to the flight crew. Such clutter is most prevalent at low altitudes and during turns when the flight crew workload is greatest. This excess clutter on the TCAS displays results in unnecessary and increased flight crew workload.
The Minimum Operational Performance Standards (MOPS) for Traffic Alert and Collision Avoidance System (TCAS) Airborne Equipment, manual document no. DO-185 by the Radio Technical Commission for Aeronautics (RTCA) which governs the operation of aircraft collision avoidance apparatuses, suggests an algorithm for merging split tracks based on multipath replies, insufficiently suppressed Mode S transponders, false replies, or other replies not accurately representative of target aircraft. This merging (in addition to enhanced merging functions) is performed in the TCAS surveillance functions. However, although some merging can be done in surveillance processes, the criteria need to be conservative so that tracks representative of real target aircraft, as opposed to a track of a split track for a single target aircraft, are not eliminated. If a track of a real target aircraft is eliminated, such target aircraft tracks are no longer tracked or updated. A threat from such an aircraft cannot be immediately introduced to the flight crew by means of the TCAS display. It takes a few seconds to re-acquire a track once it is eliminated and it takes even longer for that track to stabilize. Thus, there is some time delay danger in not adequately monitoring real target aircraft if conservative criteria for track merging in a TCAS surveillance process or collision avoidance system (CAS) process are not utilized. Such loss of track information regarding a real target aircraft which presents a threat to the surveillance aircraft is unacceptable. Therefore, there is a need to provide further TCAS track merging capabilities to remove excess display clutter while maintaining a conservative approach to eliminating tracks which are potentially split tracks based on multipath replies, false replies, etc.